Detector arrangement

ABSTRACT

The invention relates to a detector arrangement for the conversion of electromagnetic radiation into electrical signals. The detector arrangement includes sensitive areas (D 1 , D 2 , D 3 , D 4 ), where each sensitive area corresponds to a respective electrical signal, and at least two of the sensitive areas mesh with one another in such a manner that non-overlapping envelopes (C 1 , C 2 , C 3 , C 4 ) of the individual meshing sensitive areas also mesh with one another.

The invention relates to a detector arrangement which is intended toconvert electromagnetic radiation into electrical signals and includessensitive areas.

Detector arrangements which convert electromagnetic radiation intoelectrical signals are used in a variety of applications, for example,in CCD cameras or in X-ray detectors. Because detector arrangements ofthis kind are often manufactured in an integrated process, the sensitiveareas (for example, photodiodes) have essentially a simple geometricalstructure, for example, a rectangular shape, so that read-out leads andsupply leads can be readily realized between the sensitive areas. Adetector arrangement of this kind is usually intended to generate, byway of a source of electromagnetic radiation, a projection of an image(object image) which is converted into a measured image (data image) bythe detector arrangement. The detector arrangement absorbs theelectromagnetic radiation and converts it, directly or indirectly, intoelectrical signals (typically charge signals, current signals and/orvoltage signals). As a result of the sensitive areas, localized scanningof the object image takes place. Ultimately, the data image (displayimage) is presented on a display medium, for example, a monitor, thatis, possibly after post-processing of the data; the display image thenconsists of a discrete number of pixels, typically arranged in a matrixstructure of n×m pixels, where n and m are integers. A pixel is usuallyassociated with a sensitive area or with a group of sensitive areas, butthe display image matrix may also be calculated by interpolation of thedata image values. For example, the matrix of rectangularly arrangedpixels can be calculated from a matrix of hexagonally arranged detectorelements.

Detector arrangements of this kind are important in particular formedical imaging. When a detector arrangement of this kind consists of ascintillator layer and photodiodes which are situated therebelow,incident X-rays are converted into optical light quanta by thescintillator layer; the photodiodes, constituting the sensitive areas inthis case, absorb the light quanta and charge carriers are generated.Because the process of converting X-rays into light quanta in ascintillator layer is a process in which the light quanta generated areisotropically emitted from the location in which they are generated, aphotodiode cannot be unambiguously associated with a part of the surfaceof the scintillator layer. As a result, the object function of a sharpedge is not represented by a sharp “staircase structure” in the pixelsof the display image, but by a “soft” edge structure; the “softness” ofthe edge, that is the width of the edge, is then dependent on the sizeof the sensitive areas and on the thickness and the type ofscintillation material (for example, chemical composition, specificdensity, structuring) and further variables. Such softening (or low-passfiltering) of an object function by the detector is also described bythe modulation transfer function (MTF) of the detector (electroniceffects being ignored in this context). The “softening” leads to areduction of high frequencies in the object function.

In the case of a spatially discrete scanning of the object function themodulation transfer function imposed by the finite dimensions of thesensitive areas of the photodiodes is strongly limited in thereproduction of high frequencies. When the object function of a sharpedge is scanned by such a photodiode arrangement, the high frequenciesof the object function lead to disturbing aliasing effects. Ascintillator layer significantly reduces aliasing effects due to the“softening” of the incident object function, that is, by suppressing thehigh frequencies. The low-pass filtering of the relevant objectfunction, caused by the scintillator and taking place prior to thediscrete scanning by the photodiodes of finite dimensions, is alsoreferred to as pre-sampling filtering.

If the detector arrangement does not include a scintillator layer whichsuppresses the high frequencies of the object function already beforethe sensitive areas of the detector elements, disturbing aliasingeffects occur. One example in this respect is an X-ray detector whichconsists of a directly converting material and comprises anon-structured top electrode at the X-ray side and single electrodes,constituting the sensitive areas in the present example, on the lowerside of the directly converting material. Incident X-rays are absorbedby the directly converting material so as to be converted into chargecarriers. The charge carriers are accelerated by means of a strongelectrical field between the top electrode and the single electrodes soas to be converted into an electronic current signal. The strongelectrical field between the top electrode and the single electrodesensures that only little crosstalk occurs between the detector elements.Thus, the pre-sampling filtering which enhances the imaging propertiesis essentially absent in this case.

It is an object of the present invention to improve a detectorarrangement for the conversion of electromagnetic radiation intoelectrical signals in such a manner that aliasing effects are reduced.

This object is achieved by means of a detector arrangement for theconversion of electromagnetic radiation into electrical signals whichincludes sensitive areas, each of which corresponds to a respectiveelectrical signal, it being arranged that at least two of the sensitiveareas mesh with one another.

This is particularly advantageous for detector arrangements withoutpre-sampling filtering in which, therefore, the sensitive areas defineessentially the sampling of the object function. However, when thesensitive areas mesh with one another, there will be parts of asensitive area which not only adjoin parts of another sensitive area,but partly or completely enclose such parts or are enclosed thereby, orthere will be parts of sensitive areas which are enclosed completely byparts of another sensitive area. Such meshing leads to less sharpconfinement of the sampling and hence to a reduction of the aliasingeffects.

Claim 2 discloses a particularly advantageous embodiment of theinvention in which the meshing of the sensitive areas with one anotheris realized by means of dentation (coherent meshing) or by interleaving(incoherent meshing).

In respect of their sampling properties for the object function thesensitive areas are defined in conformity with claim 3, that is, by wayof respective associated sensitive surfaces which mesh with one another.

Sensitive areas which are sensitive either to electromagnetic radiationor to the conversion products of a conversion layer which forms part ofthe detector arrangement are advantageously realized way of photodiodesor electrodes as described in claim 4. In these cases the surfaces ofthe sensitive areas define the sampling properties of the sensitiveareas.

Claim 5 discloses an advantageous embodiment of the invention whichcomprises sensitive areas of equal size. This results in homogenizationof the detector properties (for example, signal level, dark currentetc.). It is also advantageous when the shape of the sensitive areasvaries, so that moiré like effects can be avoided (claim 6).

The invention also relates to an X-ray apparatus, notably an imagingX-ray apparatus, in which the detector arrangement in accordance withthe invention is employed.

The invention also relates to a method of detecting electromagneticradiation.

Several embodiments of the invention will be described in detailhereinafter with reference to the Figures. Therein:

FIG. 1 is a diagrammatic representation of an arrangement of surfaceswhich are associated with the sensitive areas in conformity with thepresent state of the art,

FIG. 2 is a diagrammatic representation of an arrangement of sensitivesurfaces which mesh with one another,

FIG. 3 is a diagrammatic representation of an arrangement of foursensitive areas with a multiple dentation of average depth,

FIG. 4 is a diagrammatic representation of an arrangement of foursensitive surfaces with a multiple dentation of larger depth,

FIG. 5 is a diagrammatic representation of an arrangement of foursensitive surfaces with a dentation with pointed tooth elements,

FIG. 6 is a diagrammatic representation of four sensitive surfaces witha dentation which is not regular but exhibits deviations,

FIG. 7 is a diagrammatic representation of an arrangement of ninesensitive surfaces whose sensitive surfaces are interleaved,

FIG. 8 is a diagrammatic representation of nine sensitive surfaces whichmesh with one another,

FIG. 9 shows an X-ray apparatus in which a detector arrangement inaccordance with the invention is employed, and

FIG. 10 is a cross-sectional view of the basic construction of adirectly converting detector.

FIG. 1 shows a 2×2 arrangement of four flat sensitive areas P1, P2 whichare denoted by shading and are known, for example, from their use in aflat X-ray detector (see, for example, U.S. Pat. No. 6,021,173 A) inwhich the sensitive areas are realized by way of photodiodes. Thesurfaces P1, P2 associated with the sensitive areas are referred to asthe sensitive surfaces. An arrangement of this kind may form part of alarger matrix. The sensitive surfaces of the present embodiment areformed by photodiodes. Even though the detection process in thephotodiode is characterized by a given absorption depth, the samplingproperties are defined by the sensitive surface of the photodiodes.

With each photodiode in the present embodiment of a flat X-ray detectorthere is associated a switching transistor which propagates the chargestored in the capacitance of the photodiode during the read-outoperation. This switching transistor is realized each time in the free(not-sensitive) area S1, S2 associated with the photodiode. In additionthere are provided (not shown) read-out leads and bias voltage leadswhich extend between the photodiodes. Because of the leads extendingbetween the photodiodes, it is difficult to indicate spatially separateareas which can be unambiguously associated with a detector element,since, for example, the line driver leads are associated with manydetector elements in common. Therefore, the subdivision into rectangularareas C1, C2 as denoted by heavy lines need not necessarily beunderstood to be a physical subdivision into detector elements. Thesubdivision, however, shows that the detector surface can be subdividedinto convex (in this case rectangular) elements in such a manner thateach convex area C1, C2 encloses the entire sensitive surface of adetector element. In this context the term “detector element” is to beunderstood as the combination consisting of the sensitive area and theassociated electronic circuitry. A convex subdivision into its elementscan also be found for detectors whose detector rows or columns areoffset relative to one another or for a hexagonal detector elementarrangement. It is to be noted that the sensitive areas shown in theFigures may also be situated on a detector surface which is curved inspace, for example, as is the case for detector arrangements forcomputer tomography apparatus which are arranged around the focus of theradiation source as the center on a segment of the surface of acylinder. Curvature in both dimensions (for example, like a segment of aspherical surface) is also permissible.

When such a detector is read out, an electrical signal is read out fromeach detector element (for example, from a sample-and-hold stage). Eachof the electrical signals corresponds to a respective sensitive surface.This results in a two-dimensional data image which corresponds to theobject image MTF filtered by the detector.

In a detector which is based on optically encapsulated scintillatorcrystals (for example, scintillator crystals separated from one anotherby means of reflectors), the scintillator crystals do not serve for thepre-sampling filtering as is the case in a non-structured scintillator.In this case pre-sampling filtering can also be realized by the meshingof the scintillator crystals which are optically encapsulated from oneanother. The sensitive areas are then formed by the scintillatorcrystals and the meshing can be configured so as to bethree-dimensional.

FIG. 2 shows an arrangement of four sensitive surfaces of detectorelements in a 2×2 configuration. The sensitive surfaces D1, D2, D3, D4of the individual detector elements are denoted by shading. The entiresensitive surface of the fourth detector element (situated at the bottomright) is now subdivided into four separate sensitive sub-surfaces D4.1,D4.2, D4.3, D4.4 which are separated, for example, formanufacturing-technical reasons from one another on the surface but areconnected to one another in deeper layers. The sensitive area of thefourth detector element is thus formed by the total area of thesensitive sub-surfaces D4.1, D4.2, D4.3, D4.4. When such a detectorelement is read out, therefore, no electrical signal is associated withjust one sensitive sub-surface; instead there is only one electricalsignal which is associated with the entire sensitive surfaceD4=D4.1+D4.2+D4.3+D4.4. The geometrical shape of the sensitive surfacesis the same for each detector element (apart from the subdivision into aplurality of sensitive sub-surfaces as shown for the fourth detectorelement) and the arrangement shown may form part of a larger detectormatrix. The heavy lines denote a gapless decomposition of the detectorsurface in envelopes C1, C2, C3, C4 of the sensitive surfaces D1, D2,D3, D4; each envelope encloses the entire sensitive surface of a singledetector element, that is, exclusively the sensitive surface of thisdetector element. Because the individual sensitive surfaces of thevarious detector elements comprise convex projections which mesh eachtime with concave indentations of the sensitive surface of a neighboringdetector element (i.e. interlocking, in this case meshing, of thesensitive surfaces of neighboring detector elements is obtained), theenvelopes can no longer be shaped so as to be purely convex.Decomposition into overlap-free convex envelopes, as shown in FIG. 1, isno longer possible in this example. If the sensitive surfaces shownherein are, for example, the single electrodes in a directly convertingdetector, as a result of the dentation the sensitive surfaces in thedentated areas distribute the image of a hard edge among the twodetector elements, the image thus undergoing low-pass filtering so thatthe desired effect a reduction of aliasing is achieved. The sensitivesurfaces are again situated on a rectangular grid (denoted by the dashedlines). However, this is not to be considered a limitation, since othergrids, for example, hexagonal or irregular grids, are also understood tobe covered by the general concept of the invention.

As opposed to, for example, a capacitive coupling between neighboringsensitive surfaces, the meshing ensures that the sharing of signalcontributions occurs only in the dentated areas, whereas a capacitivecoupling mixes the signals from the entire sensitive surface ofneighboring detector elements. The latter corresponds to a filteringafter the sampling by means of the sampling function of the object imagewhich is given by the sensitive surfaces, so that aliasing effectscannot be reduced.

FIG. 3 and FIG. 4 show further examples of different dentations ofneighboring sensitive surfaces. Only the envelopes which follow thesensitive surfaces are now shown. It is to be understood that thesensitive surfaces follow the dentations of the envelopes as representedin FIG. 2 by the shaded sensitive surfaces, that is, a convex projectionof the envelope also follows a convex projection of the sensitivesurfaces in such a manner that when the envelopes mesh the sensitivesurfaces also mesh. FIG. 3 shows a dentated structure which is morecomplex than the dentation shown in FIG. 2, and FIG. 4 shows a dentationwhich is deeper than that in FIG. 3.

FIG. 5 shows envelopes of meshing detector elements, the dentation beingrealized by way of triangular teeth. The sensitive surfaces (not shown)should again follow these envelopes, so that the sensitive surfacescomprise corresponding triangular teeth. Evidently, arbitrary othergeometrical shapes can also be used for the dentation, for example,trapezium-like teeth, triangular teeth with rounded tips, semi-circularteeth, dentations which follow a sine curve, etc. Furthermore, alldentations need not be the same in all cases; different forms ofdentation can be used on different sides of the detector elements, orthe dentation geometry may change in one position or there may bedifferent concurrent tooth geometries. The number of teeth may also besubstantially larger than shown, or there may be teeth with barbs orwidened portions (for example, as in the case of interlocking parts of apuzzle) or there may be further side teeth.

FIG. 6 shows the envelopes for a further dentation configuration. Theshape of the dentation is changed between different neighboring detectorelements, which means that the geometrical shape of the envelopes doesnot remain the same but changes. Such a change may take place gradually(that is, with only slight changes from one detector element toanother), or in a pronounced fashion; such changes may be accidental ormay follow a pattern or obey a rule. An additional secondary conditionthat the overall surface of the envelope or the sensitive surfaceenclosed by the envelope should remain constant could also be imposed.Such a change of the dentation geometry is advantageous so as to avoidmoiré-like effects which are due to regular structures in the imagesignal, for example, from regular line structures.

FIG. 7 shows an arrangement of nine detector elements in a 3 ×3configuration. Instead of using dentations, the interlocking is achievedby means of interleaved sensitive surfaces. For the sake of clarity thevarious sensitive parts of the surface which belong to the centraldetector element are denoted by shading. As for the dentated detectorelements, sampling at the area of the neighboring detector element isachieved by interleaving of the sensitive surfaces. It is to beunderstood that the connection between the sensitive parts of thesurface associated with one detector element is realized, for example,in deeper metal layers. Such a connection V1 of the surface parts whichare not coherent on the surface is shown in dashed form, by way ofexample, for the sensitive surface parts D1.1 and D1 .3. Theinterleaving is achieved in that the sensitive surfaces comprise freeareas 720 in which sensitive surface parts 710 of neighboring detectorelements are realized. Adjacent the large sensitive surface part D1.1 ofthe central detector element in this embodiment there are situatedfurther sensitive surfaces D1.2, D1.3 which belong to this detectorelement and are realized in corresponding free areas of the sensitivesurfaces D2.1 of neighboring detector elements. Analogously, thesensitive surface part D1.1 of the central detector element includesfree areas in which sensitive surface parts D2.2 of neighboring detectorelements are realized. As opposed to the meshing by way of teeth, asignal can thus be extracted from the next detector element but onebecause at that location there is a free area in which a sensitivesurface part is realized which is connected to the corresponding nextneighbor but one. As shown in FIG. 7, there is overlap between each ofthe sensitive areas to allow the sensitive surface parts 720 to bepositioned in the free areas 710. In this example of engagement it isagain impossible to realize a gap-free subdivision of the detectorsurface into convex envelopes which enclose each time only all sensitivesurface parts of a detector element.

As for the dentations, it is to be understood again that theinterleaving can be carried out by way of free areas of differentgeometrical shape and sensitive surfaces of neighboring detectorelements which are realized therein, and also the type of interleavingmay vary etc. Furthermore, dentation and interleaving may also becombined.

FIG. 8 shows interleaving in the case of 3×3 arrangement of detectorelements in which there are no fully enclosed free areas in thesensitive areas of the detector elements, but cut-outs in whichsensitive surface parts of neighboring detector elements are realized.This is illustrated for the central detector element whose sensitivesurface parts are denoted by shading. If the free areas of neighboringsensitive surfaces were shifted relative to one another, there wouldeach time be a respective envelope for the sensitive surfaces of thedetector elements in such a manner that the dentations would comprisebarbs, that is, in such a manner that two toothed detector elementscould no longer be pulled apart within the detector surface. Thisillustrates the smooth transition between dentation and interleaving.

FIG. 9 shows a typical imaging X-ray system in which a detectorarrangement in accordance with the invention can be used. It includes anX-ray source RS which exposes a patient who is arranged on the patienttable PT to emitted X-rays. The radiation that is not absorbed by thepatient table PT and the patient is converted into an image by the X-raydetector XD, which image can be displayed, for example, on a monitor orbe entered into a hospital data base system.

FIG. 10 is a diagrammatic cross-sectional view of the three essentialelements of a detector comprising a directly converting material. At theX-ray entrance side there is situated a non-structured top electrode TEwhich is deposited on the directly converting material DC (for example,CZT, cadmium zinc telluride, CdTe, PbO, PbI₂, HgI₂ or amorphousselenium, a-Se). On the lower side of the directly converted materialthere are situated the single electrodes DE. Between the top electrodeTE and the single electrodes DE there is applied a voltage U such thatcharge carriers generated by the absorption of X-rays are acceleratedand produce an electrical signal; a slight lateral drift of the chargecarriers now hardly contributes to pre-sampling filtering. A currentflow then arising can be stored, for example in a capacitance so as tobe read out at a given rate.

1. A detector arrangement for the conversion of electromagneticradiation into electrical signals, which arrangement includes sensitiveareas, each of which corresponds to a respective electrical signal,wherein at least two of the sensitive areas mesh with one another bypositioning a portion of one of the sensitive areas in a fully enclosedfree area of another one of the sensitive areas.
 2. A detectorarrangement as claimed in claim 1, wherein at least two of the sensitiveareas mesh with one another by at least one of interleaving anddentation.
 3. A detector arrangement as claimed in claim 1, whereinsampling properties of the sensitive areas are defined by a respectiveassociated sensitive surface and that meshing is realized by way of thesensitive surfaces.
 4. A detector arrangement as claimed in claim 1,wherein the sensitive areas are formed by at least one of photodiodes orelectrodes.
 5. A detector arrangement as claimed in claim 1, wherein atleast a portion of each the sensitive areas overlaps a portion ofanother sensitive area.
 6. A detector arrangement as claimed in claim 1,wherein the shape of the sensitive areas varies.
 7. An imaging X-raysystem which includes a detector arrangement as claimed in claim
 1. 8.The detector arrangement of claim 1, wherein at least a portion of eachthe sensitive areas overlaps a portion of another sensitive area.
 9. Thedetector arrangement of claim 1, wherein all of the sensitive areas meshwith one another by interleaving.
 10. The detector arrangement of claim1, wherein each of the sensitive areas is symmetrical.
 11. A method forthe conversion of electromagnetic radiation into electrical signals,which method comprises: emission of electromagnetic radiation by aradiation source, detection of the electromagnetic radiation by means ofa detector arrangement which includes sensitive areas, conversion of theelectromagnetic radiation into electrical signals such that thesensitive areas of the detector arrangement correspond unambiguously toa respective electrical signal, wherein at least two of the sensitiveareas mesh with one another so that a portion of one of the sensitiveareas is positioned in a fully enclosed free area of another one of thesensitive areas, and propagation of the electrical signals.
 12. Thedetector arrangement of claim 1, wherein the at least two of thesensitive areas that mesh with one another are connected to each otheralong a non-surface portion of the detector arrangement.
 13. The methodof claim 11, wherein the at least two of the sensitive areas that meshwith one another are connected to each other along a non-surface portionof the detector arrangement.
 14. The method of claim 11, wherein theshape of the sensitive areas is the same.
 15. The method of claim 11,wherein all of the sensitive areas mesh with one another byinterleaving.
 16. The method of claim 11, wherein each of the sensitiveareas is symmetrical.